1. Field of the Invention
The present invention generally relates to a semiconductor device having metal oxide semiconductor field effect transistor(MOSFET) structure, and more particularly to a method for fabricating a MOSFET having cobalt silicide metalization contact to the underlying silicon.
2. Description of the Related Art
Generally, with development of semiconductor fabrication technologies, MOSFETs are highly integrated in a semiconductor substrate. They have been essentially used as one of important elements in most digital signal processing apparatus. A MOSFET includes three terminals of source, drain and gate, and there is also provided a channel between electrically separated source and drain for transfer of carriers from source to drain, and vise versa.
In the above-mentioned MOSFET structure, with increase in integrity, there are strongly required shallow junction of the source and drain and suppression of parasitic capacitance which are both very important factors in capability of the MOSFET. In order to satisfy the above requests, there is proposed a technology forming metal silicides of compound of metal and silicon on source, drain and gate. As an advanced technology for the formation of such silicide films, there is proposed a self-aligned silicide(salicide) formation method, which simplifies steps for forming silicide layer on source, drain and gate.
FIG. 1 shows a conventional MOSFET structure in accordance with the self-aligned silicide formation method.
Referring to FIG. 1, there is provided a silicon substrate 1 in which device isolation oxides 2 are formed. Here, an active region is defined by two device isolation oxides. Gate 4 of polycrystalline silicon(hereinafeter referred to as "polysilicon") overlying gate oxide 3 is formed on a selected portion of the active region of the substrate 1. Lightly doped impurity regions 5 are selectively formed in the substrate 1 at both sides of the gate 4 by ion implantation. Spacers 6 of silicon dioxide(SiO.sub.2) are formed on side walls of the gate 4 by a known method. Thereafter, heavily doped impurity regions of source 7a and drain 7b are formed by ion implantation of high concentration and subsequent annealing. Afterwards, metal film is deposited at a selected thickness on the whole surface of the substrate 1. Thereafter, the substrate 1 is then annealed at a selected temperature, so as to react the deposited metal film with the underlying silicon, whereby metal silicide film 8 is formed. Thereafter, metal or metal compound which does not react with the underlying silicon are removed by selective etch.
The above-described self-aligned silicide formation method are useful in deep sub-micron MOSFETs.
Titanium disilicide(TiSi.sub.2) has been widely used in such MOSFET structure because of its low resistivity. However, since titanium used for the formation of the titanium disilicide has high reactivity with silicon dioxide (SiO.sub.2), it comes to be frequently reacted with the silicon dioxide, thereby forming a titanium silicide film on the device isolation oxide where titanium silicide need not be formed. The unnecessary titanium silicide film formed on the spacers or the device isolation oxide electrically connects the source, drain, and gate with each other, so that the MOSFET does not serve as switching element. In addition, since high stress by titanium silicide film causes defects to be generated in the source, drain and gate, electrical characteristic and reliability of the MOSFET are deteriorated.
In order to solve the above problems, there is proposed use of cobalt silicide for titanium silicide. The cobalt silicide however needs a large amount of silicon for the formation thereof to a desired thickness, so that it is difficult to form stable and shallow junction of source and drain. Moreover, the heating of the cobalt silicide to a temperature above about 1,000.degree. C. increases surface energy of the cobalt silicide grains. The high surface energy leads to an agglomeration of large silicide grains during the annealing process. As a consequence, the cobalt silicide film formed in accordance with the conventional method has a low flatness, so that its contact resistance with the underlying silicon increases.